feat(tests): Add test_mesh tool for OBJ loading and normal visualization

Implemented a new standalone test tool 'test_mesh' to:
- Load a .obj file specified via command line.
- Display the mesh with rotation and basic lighting on a tiled floor.
- Provide a '--debug' option to visualize vertex normals as cyan lines.
- Updated asset_packer to auto-generate smooth normals for OBJs if missing.
- Fixed various WGPU API usage inconsistencies and build issues on macOS.
- Exposed Renderer3D::GetVisualDebug() for test access.
- Added custom Vec3 struct and math utilities for OBJ parsing.
This tool helps verify mesh ingestion and normal computation independently of the main demo logic.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/3d/object.h b/src/3d/object.h
index 0c8edd8..a7ce0b8 100644
--- a/src/3d/object.h
+++ b/src/3d/object.h
@@ -40,11 +40,12 @@ class Object3D {
   bool is_static;
 
   AssetId mesh_asset_id;
+  void* user_data; // For tool-specific data, not for general use
 
   Object3D(ObjectType t = ObjectType::CUBE)
       : position(0, 0, 0), rotation(0, 0, 0, 1), scale(1, 1, 1), type(t),
         color(1, 1, 1, 1), velocity(0, 0, 0), mass(1.0f), restitution(0.5f),
-        is_static(false), mesh_asset_id((AssetId)0) {
+        is_static(false), mesh_asset_id((AssetId)0), user_data(nullptr) {
   }
 
   mat4 get_model_matrix() const {